This invention relates to antenna elements for radiating multiple polarization, and particularly to such elements wherein the polarization may be selected by control of wave energy signals supplied to the element.
Multiple polarization antenna elements are usable in a variety of antenna systems. In some cases where a broad angular radiation pattern is desired such an element may be used as the antenna itself. The capability to radiate multiple selectible polarizations allows matching of the antenna element polarization characteristics to those of radiation signals, thereby providing a most efficent transfer of electromagnetic radiations into electrical signals. In other cases such antenna elements may be used to illuminate parabolic reflectors in order to form a narrow beam of radiated electromagnetic energy. In still other applications such antenna elements may be arranged in the form of an array antenna and their radiated signals combined in phase to form a concentrated beam of radiation.
Many multiple polarization antenna elements use radiation in orthogonal linear polarizations, with selected amplitudes and phases to form the desired polarization. It is well known in the art that any arbitrary polarization, including right or left hand circular polarization, and any orientation of linear or elliptical polarization may be achieved by combining orthogonal linear polarization signals with selected amplitude and phase. It is desirable in such multiple polarization antennas that the radiation pattern in each orthogonal polarization be substantially identical. It is well known that the radiation amplitude pattern characteristics of a dipole radiator in the E plane are similar to the H plane radiation characteristics of a waveguide radiator. Conversely the H plane pattern characteristics of a dipole radiator are similar to the E plane radiation characteristics of a waveguide radiator. The combination of a waveguide radiator and dipole radiator therefor provides well matched spacial amplitude radiation patterns of orthogonally polarized radiated signals. Because of the spatially matched amplitude radiation pattern, the combination of these elements therefore can provide an efficient multiple polarization radiating element with good polarization control over substantially the entire portion of space into which the element radiates.
Prior art elements for radiating multiple polarizations have included waveguide radiators with parasitic dipole radiators. In such an arrangement a waveguide radiator is used to radiate wave energy in a first polarization. A dipole radiator is arranged across the wave-guide aperture and is coupled to the signals radiated by the waveguide radiator and thereby caused to radiate an orthogonal polarization. The dipole radiator in this prior art antenna is not independently supplied with wave energy signals, but derives signals from the field created by the waveguide radiator. The dipole is therefore referred to as a "parasitic dipole". The amplitude and phase of energy radiated by the parasitic dipole element is dependent on its location and orientation in the aperture of the waveguide radiator and its radiation consequently has a fixed amplitude and phase with respect to the radiation from the waveguide radiator at any particular frequency. It is consequently not possible to change the radiated polarization of the combined element by variation of the amplitude or phase of the supplied signal.
Other prior art systems may have used dipole radiators in combination with waveguide radiators. In accordance with such prior art it has not been possible to locate the radiation phase center of the dipole radiator at the radiation phase center of the waveguide radiator because of potential interference of the structures with electromagnetic fields.
It is therefore an object of the present invention to provide an antenna element for radiating signals in multiple polarizations.
It is a further object of the present invention to provide such an antenna element wherein the polarization of the radiated signal is variable in accordance with the relative phase and amplitude of supplied energy wave signals.
It is a still further object of the present invention to provide such an antenna element wherein the radiated signals in each of orthogonal linear polarizations are radiated with the same phase center.
It is a still further object of the present invention to provide a multiple polarization array antenna system.